1. Field of the Invention
The present invention relates to an electronic endoscope which comprises a solid image capturing element and which can be used with various treatment tools.
2. Description of the Related Art
As is well-known, endoscopes enable, for example, the interior of a living organism, which is not directly visible, to be observed and have been widely used for diagnosis and treatment mainly in medical fields. Further, electronic endoscopes have been prevailing which convert a subject image into an electric signal using a solid image capturing element such as a CCD so that the image can be observed via a monitor. In recent years, the following have also been spreading: electronic endoscopes that employ a zoom optical system in order to closely observe a subject and high-resolution endoscopes that use a multi-pixel solid image capturing element.
The former electronic endoscope employing a zoom optical system cannot adopt a complicated configuration owing to a limitation on the increased size of configuration of a distal end. Accordingly, resizing zoom optical systems are commonly used which move one lens group to vary the view angle.
Such a resizing zoom optical system as shown in Japanese Patent Laid-Open No. 2000-330019 is composed of a first lens group 10 having a negative refractive power, a brightness aperture S, a second lens group 20 having a positive refractive power, and a third lens group 30 having a negative refractive power; these groups are arranged in this order from an object as shown in FIG. 1 of this publication. This system is characterized in that for resizing, the second lens group 20 moves to two different points on an optical axis which do not vary an inter-object-image distance with the first lens group 10 and third lens group 30 immobilized. G denotes filters.